The present invention relates to golf balls made using a polyurethane/ionomer resin blend which has a good heat resistance, good moldability, good paint film adhesion and good adhesion to adjoining layers, and from which can be obtained high-performance golf balls endowed with an excellent rebound and excellent feel on impact. The invention relates also to a method of manufacturing such golf balls.
Wide use has hitherto been made of ionomer resins in golf ball cover stock compositions. Ionomer resins are ionic copolymers composed of an olefin such as ethylene in combination with a metal salt of an unsaturated carboxylic acid such as acrylic acid, methacrylic acid or maleic acid. The acid groups in the copolymer are partially neutralized with metal ions such as sodium or zinc ions. These resins have excellent characteristics such as durability and rebound, and are suitable as the base resin in golf ball cover stock.
When an ionomer resin is used as the cover stock in a golf ball, the cover of the ball is very hard and lacks a flexibility sufficient to impart the spin rate necessary for controlling the ball in flight. Moreover, golf balls that are made using a hard ionomer resin as the cover stock have a poor “feel” at the time of impact.
To overcome such drawbacks of ionomer resins, cover stocks using thermoplastic polyurethane elastomers were proposed in U.S. Pat. No. 3,395,109, U.S. Pat. No. 4,248,432, and U.S. Pat. No. 4,442,282. Cover stocks using thermoplastic polyurethane elastomers provide a better feel when the ball is played and better controllability than do cover stocks made with ionomer resins.
More recently, U.S. Pat. No. 4,674,751 teaches a cover stock which is composed of a blend of an ionomer resin with a polyurethane elastomer and which provides a good compromise between the resilience of an ionomer resin-based cover stock and the feel of a polyurethane elastomer-based cover stock.
However, the ionomer resin and the polyurethane elastomer are of such a nature as to form chemical bonds when mixed under heating. When the cover stock composition is kept hot within a processing apparatus, it undergoes gelation and becomes impossible to mold, which is impractical.
I earlier found that adding an organic or inorganic basic compound to the polyurethane/ionomer resin blend is highly effective for preventing the polyurethane/ionomer resin blend from gelling when kept hot, and thus disclosed the golf ball material described in U.S. Pat. No. 6,822,028. This golf ball material does not gel during molding, and golf balls manufactured using the material exhibit a good rebound energy.
However, when such a golf ball material is used, because it contains organic bases such as amines that have a corrosive effect on the mold used to manufacture the golf ball, it leaves something to be desired in terms of the molding conditions and, depending on the equipment used, in terms of safety.
Also, because most such organic bases are low-melting aliphatic amines, they readily migrate to the surface of the molded article, compromising adhesion with adjoining layers and thus lowering the durability of the golf ball to impact. Paint film adhesion also is less than satisfactory.